1. Field of the Invention
The present invention relates to a control system for a continuously variable transmission (CVT) in a motor vehicle.
2. Background of the Invention
A CVT is to control continuously speed ratio e (=output side rotational speed Nout/input side rotational speed Nin) to be used for a motor vehicle as a power transmission system with an excellent specific fuel consumption. In this CVT, the horsepower requirement is set as a function of throttle position .theta., and the engine speed Ne meeting each horsepower requirement with the minimum fuel consumption is defined as the desired steady-state engine speed in the throttle position .theta.. In the prior art CVT, the desired engine speed is not particularly set in a transient time, but equalized to the desired steady-state engine speed. In this case it takes time until the actual engine speed reaches the desired steady-state engine speed and inconvenience was found in slow responsive property in the transient time. Thus, the assignee of the present application proposed a control system for the CVT in the prior patent application Sho No. 58-17550 filed in the Japanese Patent Office on February, 1983 so as to eliminate such inconvenience. According to this control system, the desired engine speed is set as a function of the throttle position and the engine speed or the speed ratio of the CVT is controlled such that the actual engine speed reaches the desired engine speed. Rate of step change B is defined as a positive number less than 1. When the throttle position changes substantially, the desired engine speed is discretely changed by the product B.multidot.(Nos-Nos') of a difference Nos-Nos' between the desired steady-state engine speed Nos after the change in the throttle position and the desired steady-state engine speed Nos' before the change in the throttle position and the rate of step change B. Then, the desired engine speed is gradually changed toward the desired steady-state engine speed Nos after the change in the throttle position. Namely, since the desired engine speed in the transient time is smoothly increased, the engine speed ne is speedily increased to smooth the acceleration. However, the rate of step change B to ensure a predetermined responsive property is set by this control system without distinguishing acceleration time from deceleration time so that both drivability and drive feeling were difficult to optimize both in the acceleration time and deceleration time. Namely, when a driver requests large acceleration by pedalling hard the accelerator pedal it is necessary to rapidly increase the engine speed and give large output shaft torque to the vehicle to ensure the satisfactory drivability. But, the driver does not necessarily need an abrupt speed change when he returns largely the accelerator pedal to the intial position. In the latter case, the drive feeling is improved when not only the output torque of the engine, but also the drive torque including inertial torque of the engine and CVT are smoothly changed.